This invention relates to recovery of volatile compounds from solids present in an aqueous solution. Specifically, this invention relates to processes for recovering volatile compounds with normal boiling points at or above the boiling point of water from particulate or dissolved solids present in an aqueous solution.
Over the years, there have been many technological developments in the field of aqueous waste stream processing. Environmental concerns have prompted advances in the treatment of organic compounds from aqueous waste streams as well as the substitution of organic solvents with water during synthesis. Wastewater treatment has focused on chemical or microbial reactions on aqueous waste streams to precipitate or destroy offensive compounds. For synthesis processes, the model of sequential separation of insoluble solids and water evaporation followed by purification of the organic compound has been widely practiced. Other techniques such as absorption, extraction, leaching, ion exchange, and bubble foam separation have been developed to improve operating efficiency and reduce cost during purification (Perry""s Chemical Engineering Handbook, 7th ed.; Perry, R. H. and Green, D. W., Eds; McGraw Hill: New York, 1997 (hereinafter xe2x80x9cPerry""sxe2x80x9d); Handbook of separation Techniques for Chemical Engineers, 2nd ed.; Schweitzer, P. A., Ed.; McGraw Hill: New York, 1988; Biochemical Engineering and Biotechnology Handbook, 2nd ed.; Atkinson, B. and Mavituna, F., Eds.; Stockton Press: New York, 1991; Chapter 16, FIG. 16.5).
Steam distillation or stripping has been traditionally used to purify temperature-sensitive volatile organic compounds. In steam stripping, water vapor (steam) is used to separate the volatile organic compound from less volatile compounds. The resulting products of steam stripping are aqueous heels containing less volatile compounds and volatile product with higher water content.
As described in Process Drying Practice (Cook, Edward M. and DuMont, Harman D., McGraw Hill, Inc., New York, 1991), techniques generally referred to as xe2x80x9cdryingxe2x80x9d are routinely used to preserve solids from spoilage by removal of water, to reduce weight for shipping, to reduce weight or volume for packaging requirements, to make specific shapes or uniform mixtures, to recover solvents for reuse while drying solvent slurries, to separate noxious or toxic liquid(s) from solid(s), and to remove unwanted solid(s) and to recover the liquid.
The speed of drying influences solid formation and solid product quality. Rapid drying (i.e. rapid evaporation of volatile compounds from solids) has been found to be advantageous for temperature-sensitive materials as well as for controlling solid characteristics. These rapid drying techniques include flash drying, spray drying, fluidized bed drying, and mechanically-agitated drying where residence time is minutes or seconds.
Flash drying typically involves a very short exposure (a few seconds) of a slurry to a turbulent hot gas stream. Pikkov et al. (Khim. Prom. (Moscow) 1973, 49:11 (822-3)) describe a device for isolating glycerol from the liquid hydrogenolysis products of sucrose. The device contains a venturi nozzle through which superheated steam is injected and an inlet for the hydrogenolyate, which is fed at right angles to the steam jet.
Spray drying typically involves exposure of the slurry to hot gas in a vertical tower for tens of seconds. The slurry may be distributed into the tower with a variety of devices including spinning disk and nozzle atomizers (Spray Drying Handbook, 5th ed.; Masters, K.; John Wiley and Sons: New York, 1991; (hereinafter xe2x80x9cMastersxe2x80x9d)). Spray drying has been extensively used in the chemical, food, pharmaceutical, and biochemical industries (Masters, supra, Part V). DD 155788 discloses a spray drying process to separate hydrocarbons and/or biological matter from a suspension of microbes grown on hydrocarbons and or other carbon sources with simultaneous cell decomposition, thus improving the biological value of the solid product. A suspension of microbes containing 17% solids, 3% hydrocarbon, and 80% water was heated to 175xc2x0 C. at 9.5 atmospheres and then pressure was reduced to 1.1 atmospheres. The resulting product had higher solids content and lower content of extractable matter, fats and fatty acids, and hydrocarbons, steroids, and phosphatides.
Fluid bed drying of a slurry allows for increased contact between the slurry and the drying gas and/or heated surface by distributing the liquid feed source over the surface of an active, churning bed of relatively dry support solid. The drying bed is typically comprised of the recycled dried solids or an inert material. Volatile product residence time can be controlled from seconds to minutes by the relative mass flow of feeds to solids in the dryer (Perry, supra, Section 17).
Mechanically agitated drying describes a broad range of techniques and equipment where the slurry or solid is transported mechanically. Residence time of the volatile product can be from minutes to hours. Equipment in this category includes rotary hearth furnaces, tunnel dryers, conveyor belt dryers, rotary (kiln) dryers, rotating plate dryers, rotating (double cone) vacuum dryers, paddle dryers, ribbon dryers, as well as other mixing equipment with internal agitators to distribute slurry or solid while volatile products are removed with a stripping agent or vacuum (Perry, supra, Section 18).
The drying techniques described above have focused on solids management and the quality of the recovered solids. In contrast solvent recovery has been considered an environmental issue as part of solid drying, or as a technique to recycle organic solvents that are contaminated with suspended or dissolved solids. The solvent recovery techniques have addressed only non-aqueous systems.
Aqueous solutions containing volatile compounds and suspended or dissolved solids (such as the products of fermentation) are processed through solid/liquid separation systems such as filters or centrifuges to manage the suspended solids before recovery of the volatile compound from water. For volatile compounds with normal boiling points below water (such as ethanol), product is recovered directly from the fermentation broth using gas stripping, evaporation under vacuum, or steam distillation.
Solids management is often labor-intensive, may require specialized equipment, and is difficult to scale-up predictably. Traditional solids management techniques have the following deficiencies:
(1) Filtering to remove insoluble solids can be problematic, especially when the solids vary in type and composition or come from biological sources such as food processing or fermentation processes.
(2) To maximize the yield of recovered volatile product while minimizing the byproduct quantity, the isolated solid is typically washed with water. Washing can compromise refiltration of the solids and adds expense and handling concerns to the down-stream process.
(3) Evaporating water from volatile product in the presence of dissolved solids can be problematic due to precipitation of the solids, fouling of heat transfer surfaces, or undesirable degradation of the volatile product.
(4) Other techniques to manage the dissolved solids (such as membrane filtration, ion exchange, or adsorption) may increase the number of operations, add supplemental materials, generate additional waste streams to be managed, and increase the complexity of material movement. Each of these techniques adds to the cost of processing.
The prior art does not teach a reliable, efficient, and economical method for recovering volatile compounds with normal boiling points at or above that of water from particulate or dissolved solids present in aqueous solutions. The problem to be solved, therefore, is to overcome the difficulties of recovering volatile compounds from aqueous solutions containing solids in a reliable, efficient, and economical manner.
A process is provided for recovering volatile compounds from solids present in an aqueous solution comprising: (a) optionally pretreating the aqueous solution with at least one pretreatment, for instance, heating, concentrating, physically altering, or adding compounds to limit undesirable reactions or conditions or to promote favorable reactions or conditions. Such pretreatment additions include adjusting pH, temperature or pressure, introducing additives to coagulate solids or to provide a support for the solids, adding salts, adding alcohol, adding minerals, adding chelating compounds, and adding buffers. Additional steps include: (b) optionally using a stripping agent to facilitate vapor removal of the volatile compound(s); (c) removing volatile compounds from the aqueous solution with rapid separation ; and (d) isolating the volatile compound(s) from the solid product of the rapid separation (c). Rapid separation may be performed by 1) flash drying, spray drying, fluid bed drying, or mechanically agitated drying, 2) gas/solid separation, and 3) cooling. Isolating may be performed by condensing, distilling, or selective scrubbing. More particularly, the process may be used to recover 1,3-propanediol or glycerol from a fermentation broth.